Like the rest of his generation, the hero of Sebastian Barry's fourth novel was born at the wrong time, in the 'dying days of the century', which made him just old enough to enlist at the outbreak of the First World War. He was also born in the wrong place, in a Dublin packed with political differences that would explode with devastating force in the Easter rising of 1916.

Right from the start, Willie Dunne is doomed; he already belongs with the nameless millions whose fate is briefly written 'in a ferocious chapter of the book of life'. And, indeed, anyone who read Barry's last novel, Annie Dunne, may recall that Annie's brother barely registered in her memories of childhood, dispatched to his muddy resting place in little more than a paragraph. Yet here, Barry sets himself the task of spinning this ephemeral life into fully fledged story, in which a scrawny, mewling baby, Willie, is greeted as 'a scrap of a song none the less, a point of light in the sleety darkness, a beginning'.

The author's determination to make something substantial of Willie Dunne is shared by the boy's father, an imposing 6ft 6in policeman who has great hopes that his son will follow him into the force. To Willie's bitter dismay, however, he never reaches regulation height and only when he goes to fight for 'King, Country and Empire' does he feel he's reached 'bloody manhood at last'. It is not long, of course, before Willie realises just how bloody his manhood will be. The intimate brutality of life in the trenches is evoked in visceral detail, from the stench of raw terror to the sensation of walking on a 'foul carpet of crushed dead'. In this landscape of death, all the normal associations of domestic and natural life are horribly mangled and imbued with a macabre grace: gas folds over the trench like a bedspread and a kingfisher shoots along the river bank like a 'glistening blue bullet'.

The poetic quality of Barry's writing, in which a description of the arrival of winter comes with three dazzling similes, may initially seem to add a layer of inappropriate luxury and beauty to the bleak subject matter, but it serves a deeper purpose here, reflecting Willie's faltering understanding of the war.

As the political and moral ground slides beneath his feet and the Irish soldiers are simultaneously despised by nationalists as traitors and denounced by the English as mutineers, Willie clutches at familiar symbols in a desperate attempt to bridge the gap between the world he knows and the one he has stumbled into.

The great achievement of this novel is the restraint with which Barry allows the awful complexity of Willie's situation to dawn on him. Early in the story, when he learns something disturbing about his father's policing, we are told that the knowledge 'sat up in Willie's head like a rat and made a nest for itself there'. During the course of the novel, the scampering of confused thought and the constant gnaw of doubt gradually become impossible for Willie to ignore.

With the grim inevitability of a tragedy, Willie is successively stripped of all that propelled him into war, from the heroic ideals invoked by Kitchener to the hand-me-down beliefs inherited from his father and his romantic hopes for the future.

Willie, and the men like him, went to war not so much to fight against the Germans, but to fight for their country, only to find that the most deadly enemy came from their own side and that the Ireland they had grown up believing in had dissolved behind them 'like sugar in the rain'. What remains for each of them is a trembling body and a head full of screams.

The title of this novel comes from the popular First World War song - 'It's a long, long way to Tipperary/ But my heart's right there!' - which presents one view of the Irish soldier's relationship to his native land. Willie Dunne's story, his 'scrap of a song', offers another perspective, one in which the only place that feels like home is a bomb-scuffed trench and the only people who really comprehend the hollowness of the heart are those who have lived it too. With disarming lyricism, Barry's novel leads the reader into a hellish no-man's-land, where the true madness of war can only be felt and understood rather than said.